The present invention relates generally to cogeneration systems of the type conventionally employing a gas turbine for generating electricity, a heat recovery steam boiler system for generating steam from the high temperature exhaust gas and an emission control system for reducing the NOX or other pollutants in the engine exhaust gas, and the present invention relates more particularly to a new and improved method and apparatus for regulating the exhaust gas temperature for the reduction of the pollutants in the exhaust gas at a preestablished desired nominal temperature.
Cogeneration plants such as a conventional gas turbine cogeneration plant employed for generating electric power and steam are being subjected to increasingly stringent NOX emission standards and such that a satisfactory emission control system is required to minimize the undesirable emissions exhausted to the atmosphere. A selective catalytic reduction system (SCR system) is presently considered by some authorities to be the best available control technology for the reduction of NOX from the exhaust gas of a cogeneration plant and as a consequence is required equipment. Currently available SCR systems used for the reduction of NOX employ ammonia injection into the exhaust gas stream for reaction with the NOX in the presence of a catalyst to produce nitrogen and water vapor. Such SCR systems typically have an efficiency of 80-90 percent when the exhaust gas stream is at temperature within an optimum temperature range of approximately 600.degree.-700.degree. F. The NOX reduction efficiency of the SCR system will be significantly less if the temperature is outside the optimum temperature range. In addition, the usual SCR system catalyst can be damaged at higher temperatures.
The engine outlet temperature of most gas turbine cogeneration plants, at full or rated load of the gas turbine engine, is conventionally between approximately 775.degree. F. and 1050.degree. F. Since the exhaust gas temperature is above the optimum temperature range of the usual SCR system, it is necessary to reduce the temperature of the exhaust gas stream before it passes through the SCR system. Current practice is to provide steam superheater and/or steam generating tubes upstream of the SCR system to withdraw heat from the exhaust gas stream to cool the gas to a preselected desired nominal temperature before it passes through the SCR system. The exhaust gas stream is then conducted through the remaining sections of the waste heat recovery boiler or other heat transfer system for generating additional steam, heating feedwater, etc. In the upstream heat exchanger, the heat transfer rate for exhaust gas stream cooling must be carefully calculated, taking into account the temperature and flow of the exhaust gas stream and the temperature and flow of the internal working fluid of the heat exchanger. Such imposes significant operating limitations on the cogeneration plant which either seriously limit the operating range of the gas turbine engine or require an undesirable exhaust gas bypass, adjustable damper or other mechanism for diverting a portion of the exhaust gas stream from the upstream heat exchanger. In addition, where supplementary firing is provided to increase steam production, the supplementary firing is conventionally located downstream of the SCR system because it heats the exhaust gas above the optimum temperature range of the SCR system. That is undesirable because it reduces the steam generating efficiency and produces additional NOX that is not removed by the SCR system. If the environmental authorities require supplementary firing to be placed upstream of the SCR system, the conventional upstream heat exchanger will not maintain SCR temperature within the optimum range throughout the full range of supplementary firing. Thus, either the gas turbine power will have to be reduced to accommodate a reduction in full load steam demand, excess steam will have to be dumped or an exhaust gas bypass, damper, etc., will have to be employed to divert a portion of the exhaust gas from the upstream heat exchanger. The disadvantages of reducing the power output of the gas turbine engine are well known and include reduced electric power generation, increased specific fuel consumption and increased gas turbine maintenance costs due to thermal cycling. Although steam dumping can sometimes be used, for example to drive a condensing steam turbine, such an approach is often impractical due to capital cost and steam turbine operation considerations. For those reasons, many gas turbine cogeneration plants currently use an exhaust gas bypass stack for controlling the heat energy content of the exhaust gas stream conducted through the heat recovery system.
It is a principal object of the present invention to provide in a cogeneration system of the type described, a new and improved method and apparatus for regulating the exhaust gas temperature for the removal of pollutants from the exhaust gas at a preselected desired nominal temperature throughout the full range of operation of the cogeneration system.
It is another object of the present invention to provide in a gas turbine electric power and steam cogeneration system, a new and improved method and apparatus for regulating the temperature of the engine exhaust gas stream for efficient reduction of certain undesirable emissions while permitting the internal combustion engine to operate at maximum efficiency and a supplementary firing system to be used to increase steam production.
It is a further object of the present invention to provide in a gas turbine engine cogeneration plant of the type having a catalytic emission control system for the catalytic reduction of certain undesirable emissions, a new and improved method and apparatus for maintaining the exhaust gas stream passing through the catalyst at a preselected desired nominal temperature providing generally optimum effectiveness of the catalytic process without employing a conventional exhaust gas bypass stack, damper or other mechanism for diverting any of the exhaust gas stream.
It is another object of the present invention to provide in a combined heat recovery and emission control system operable for recovering heat from and reducing pollutants in a high temperature exhaust gas stream, a new and improved method and apparatus for regulating the temperature of the exhaust gas stream for the efficient reduction of pollutants.
Other objects and advantages of the present invention will become apparent from the accompanying drawings and detailed description.